US Secretary of State John Kerry has said Washington “acknowledges and regrets” the fatal airstrike on Syrian government troops by coalition jets, but blamed Syrian President Assad for “bombing people indiscriminately” and blocking the delivery of humanitarian aid.

In an interview with CNN, Kerry urged Russia to pressure Syrian leader Assad, whom he called “a spoiler [who] doesn't want a ceasefire.”

“The biggest judgment they [Russia] need to make is to stop Assad from bombing people indiscriminately, which he continues to do,” the US secretary of state said.

“To allow Assad – who is a spoiler; he doesn't want a ceasefire – to allow him to continue to go after opposition, pretending that they are Nusra, is in and of itself a huge challenge to this effort [to observe the ceasefire],” Kerry said.

Last week, Moscow and Washington agreed to influence the Syrian government and the so-called moderate rebel forces respectively in order to establish a ceasefire in the country.

Washington has blamed Russia for not pressuring Damascus enough to facilitate humanitarian access to Syria. Russia has said that the US is failing to keep its part of the bargain, stating that militants are using the truce to prepare a large-scale offensive, particularly in Aleppo.

Russia's Defense Ministry on Monday said it would be “senseless” for the Syrian government to respect a ceasefire when US-backed rebels do not do the same.

“Syrian servicemen and peaceful citizens are still dying. This is because the United States has no effective leverage to influence Syria's opposition and is unaware of the real situation on the ground,” Lieutenant-General Sergey Rudskoy told a briefing.

“As the terms of the ceasefire are not respected by militants, we consider observing it unilaterally by the Syrian government forces would be senseless,” Rudskoy added.

The ministry’s monitors on the ground reportedly registered some 53 ceasefire violations of the ceasefire over the past 24 hours. They also said they received intelligence that the so-called moderate Syrian rebel units have been merging with terrorist groups in the area to prepare joint attacks.

Meanwhile, a humanitarian aid convoy consisting of 40 trucks stocked with supplies for the embattled Syrian city of Aleppo is waiting at a special customs checkpoint at the Turkish border. In his interview, the US secretary of state put the blame for the holdup on Assad and Moscow – which is supporting the Syrian leader as a legitimate authority – while US wants him stripped of power.

“The regime, once again, is blocking [humanitarian assistance]. So Russia's client, Russia's supported friend, is the single biggest blockade to the ability to move forward here,” Kerry said, adding that Russia should “stop the grandstanding, stop the showboating and get the humanitarian assistance going.”

Access to Aleppo has been blocked due to fighting over the strategic Castello Road leading into the besieged rebel-held part of the city. The road must become a demilitarized zone for aid to be transported along it, meaning both Syrian government troops and the rebels have to pull back. Moscow claimed the Syrian Army had twice withdrawn from the road already, but had to return as rebel groups had not taken the same steps.

“Twice [the Syrian troops] have withdrawn heavy weapons, equipment and personnel from the road, moved at a distance set by the [ceasefire] agreement. But due to provocations and the reluctance of the opposition [to make similar steps], the execution of this part of the arrangement was disrupted,” Rudskoy told the briefing on Monday.

“The armed forces backed by the US have failed to establish a checkpoint in the eastern part of the Castello Road and have not transferred control over it to the Syrian Red Crescent Society. They also have not yet withdrawn their troops, arms and equipment from the road,” Rudskoy said. He added that the situation looks as if it were intentionally created by the US to accuse Russia and the Syrian government of disrupting the delivery of humanitarian assistance, “although Russia has repeatedly called on all countries to join forces in conducting humanitarian operations in Syria and particularly Aleppo.”

Rebel groups however have denied reports that the Syrian Army has been pulling back from the Castello Road, putting their reluctance to move down to their wish not to be the first to leave their positions.

The convoys were still waiting at the Turkey-Syria border as of Monday morning, Reuters reported, citing UN spokesman Jens Laerke.

Speaking to CNN, Kerry also called for a “joint implementation center” to be created to help US and Russian efforts to fight terrorists in Syria become more synchronized – something Moscow has been consistently calling for since the start of its airstrikes in Syria last year. Kerry however suggested that such a facility had not been established because of Assad.

“So let me just say this clearly: Russia signed up to a cessation of hostilities. Assad said he would live by it. Then he needs to stop [blocking aid] and let the joint implementation center get set up so Russia and the United States can coordinate in order to avoid the kind of terrible thing that happened yesterday that we all acknowledge and regret,” Kerry said.

The US secretary of state was referring to the bombing of Syrian government forces’ positions near the eastern city of Deir ez-Zor that took place on Saturday and saw 62 troops killed and over 100 more wounded. Four strikes against Syrian positions were performed by two F-16 jet fighters and two A-10 support aircraft belonging to the US-led coalition fighting Islamic State (IS, formerly ISIS/ISIL) in Syria. Shortly after Syrian Army positions were attacked from the air, Islamic State launched an offensive in the area, according to Damascus.

Syrian President Bashar Assad has slammed the attack on the troops as “flagrant aggression” on the part of the US-led coalition, which has displayed “increasing support for terrorists” with this latest action.

The seven-day ceasefire brokered by the US and Russia and declared by the Syrian Army last week has meanwhile expired. No announcement of an extension has so far been made. During the “regime of calm,” the second this year, both Syrian government forces and rebels have repeatedly accused each other of violations.